1. Field of Applicable Technology
The present invention relates to a flat configuration image display apparatus for use in applications such as a color TV receiver, computer terminal, etc. In particular, the invention relates to an improved flat configuration color display cathode ray tube of the type which has a parallel array of line cathodes as an electron beam source.
2. Prior Art Technology
In the prior art, examples of a flat configuration cathode ray tube (hereinafter abbreviated to CRT) have been disclosed for example in Japanese Patent Laid-open Numbers 54-143063 and 55-33734, etc. With such a flat CRT, a set of mutually parallel thermionic line cathodes are each aligned extending horizontally (i.e. in the horizontal direction of a displayed picture) within an evacuated envelope, and are successively utilized during each scanning field to derive a corresponding horizontal row of electron beams, which are utilized in forming a set of horizontal lines of each picture field. Specifically, all of the electron beams of such a row are deflected horizontally in synchronism by a fixed amount to form one horizontal scanning line, then the beams are deflected vertically by a fixed amount and again deflected horizontally to form the next picture line, and so on. The horizontal and vertical deflection of the electron beams is executed by means of horizontal deflection electrodes and vertical deflection electrodes through which the electron beams are passed, before being accelerated to fall on a fluorescent layer formed on the inner surface of a transparent faceplate. The present invention is directed towards a color display type of flat CRT, in which the fluorescent layer consists of a pattern of fluorescent layer portions which emit respectively different colors of light, i.e. red, green and blue-emitting layer portions. This pattern will typically consist of successively alternating vertical stripes of red, green and blue-emitting fluorescent material.
A typical example of such a prior art flat configuration CRT will be described referring first to FIG. 1A. Numeral 1 denotes an array of mutually parallel thermionic line cathodes, extending horizontally and disposed at successive spacings in the vertical direction. (In the following description and in the appended claims, the designations "horizontal" and "vertical" are to be understood as referring to directions respectively parallel to the horizontal and vertical directions of a display picture produced by the CRT.) Each of the line cathodes 1 is formed of tungsten wire having a diameter that is in the range of 10 .mu.m to several tens of .mu.m, which is coated with a layer of a cathode oxide electron emission material to a thickness which is in the range of several .mu.m to several tens of .mu.m. A voltage is applied (from a source not shown in FIG. 1A) between the ends of each of these line cathodes to heat the cathode to a temperature in the range 600 to 800.degree. C. A rear electrode 2 is disposed on the opposite side of the array of line cathodes 1 from the electron beam emission side, for use in successively selecting the line cathodes 1 during each vertical scanning interval and for directing the emitted electrons of a selected cathode in the beam-emission direction. The method of heating and biasing each of the line cathodes 1 is illustrated in FIGS. 2A and 2B. As shown in FIG. 2A, each of the line cathodes 1 is coupled at one end through a corresponding diode 21 to ground potential. A corresponding drive voltage V.sub.c is applied to the diode from the aforementioned source, with the waveform of this drive voltage being as shown in FIG. 2B. Normally, each of the line cathodes 1 is held at a positive bias voltage E.sub.k, to thereby heat the line cathode to the requisite temperature by a current which passes through the diode 21. In this condition, emission of electrons is inhibited. However during a specific part of each vertical scanning interval in which that line cathode is utilized (for example during an interval in which 16 successive horizontal picture lines are generated by means of a row of electron beams derived from that line cathode) a negative bias voltage E.sub.kp is applied to the line cathode, thereby reverse-biasing the diode 1a to thereby interrupt the flow of heating current and also setting the line cathode to a uniform negative potential, thereby enabling electron emission therefrom. The intervals in which this negative bias is applied occur sequentially during each vertical scanning interval, for successive ones of the line cathodes. The back electrode 2 can be formed as a metal plate, or as a conducting layer that is formed on an interior surface of the evacuated envelope of the CRT (not shown in the drawing).
Numeral 3 denotes an electron beam forming electrode, for forming a plurality of electron beams 11 from electrons that are emitted from the line cathodes 1. The beam forming electrode 3 has horizontal rows of through-holes 3a formed therein for passage of the electron beams 11, with the rows of through-holes 3a being disposed respectively opposite the line cathodes 1. Successive rows of electron beams are thereby generated from the line cathodes during each vertical scanning interval The shape, dimensions, and numbers of the through-holes 3a are determined by the requisite number of electron beam spots and the amplitude of the electron beam current, etc. A set of vertical deflection electrodes 4 can consist for example of a set of electrically conducting regions 4a that are each formed on a surface of a corresponding electrically insulating substrate. The vertical deflection electrodes 4 are driven by scanning voltages to deflect the electron beams 11 vertically. Numeral 5 denotes a set of modulation electrodes having vertically elongated slits 17 formed therein through which respective ones of the electron beams 11 pass, for controlling the intensities of the electron beams 11 in accordance with respective voltage signals which are applied to the electrodes 5 in accordance with the image display contents.
A shield electrode 6 has vertically elongated apertures 18 formed therein, corresponding in position to the apertures 17 in the modulation electrodes 5, and serves to provide shielding between the electrodes which are disposed before and behind the shield electrode. Horizontal deflection is executed by two electrically separate comb-shaped horizontal deflection electrodes 7a, 7b, which are meshed such as to form vertically elongated apertures 20 which are respectively positioned in correspondence with the apertures 17, 18 of the modulation electrodes 5 and shield electrode 6, i.e through which each of the electron beams 11 passes as illustrated in FIG. 1A, to be deflected in the horizontal direction. The vertically extending "teeth" portions of each of these electrodes are mutually connected to receive deflection voltages, with these connections being indicated as bus leads 27a, 27b in FIG. 10. Numeral 8 denotes a set of acceleration electrodes, for accelerating the electron beams 11, and 9 denotes a transparent faceplate of the CRT (formed of a material such as glass). In general, the transparent faceplate 9 is part of the envelope of the CRT, and has a light emission layer 10, including a fluorescent layer 10a formed on the inner surface thereof and with a metal back layer 10b (e.g., a thin film of aluminum) formed over the fluorescent layer. A high voltage (e.g. 5 to 20 KV) is applied to the metal back layer, identical to a voltage which is applied to the acceleration electrodes 8. In the case of a color display CRT, the fluorescent layer 10a consists of a pattern of fluorescent layer portions, e.g. consisting of alternating red, green and blue-emitting stripes as mentioned above.
The position relationships between the aforementioned gaps and apertures through which the electron beams 11 pass are illustrated in the partial plan view of the prior art CRT of FIG. 1A shown in FIG. 1B. Respective central axes (i.e. each passing centrally through an aperture in a direction perpendicular to the fluorescent layer 10a ) of a set of apertures 17, 18 and 20 (of the modulation electrodes 5, shield electrode 6 and the horizontal deflection electrodes 7a, 7b) lie in a common straight line. As viewed in plan, this line corresponds to the trajectory of an electron beam 11 when in the horizontally undeflected condition.
Such a prior art flat configuration color display CRT has the advantages of a simple configuration, high brightness, and high resolution, together with a thin overall shape. However it has the practical disadvantage that it requires an extremely high accuracy of mutual position alignment between the fluorescent layer 10a pattern and the horizontal deflection electrodes, so that assembly of the CRT is difficult and only a relatively low manufacturing yield is attainable. The reasons for this will be described referring to the frontal view of FIG. 3, in which the fluorescent layer 10a consists of a pattern of fluorescent layer stripes for red, green and blue emission (designated as R, G and B), alternatingly arranged and extending vertically, with the horizontal deflection electrodes 7a, 7b shown in broken-line outline. The most serious form of position deviation between the horizontal deflection electrodes and the fluorescent layer stripe pattern is that shown in the drawing, with the horizontal deflection electrodes being slightly rotated (from a correct position) about a central point thereof with respect to the fluorescent layer stripes. With such a condition, accurate color rendition of a display picture becomes impossible. Specifically, assuming for example that a specific horizontal deflection condition (e.g. with no horizontal deflection being produced, so that each electron beam passes through the center of a corresponding one of the gaps 20 in the horizontal deflection electrodes) is being applied to the electron beams 11 when an uppermost horizontal picture line is being generated, and that an electron beam is falling on a specific fluorescent layer color stripe in that condition, it will be apparent that for an identical horizontal deflection condition when a horizontal picture line is being generated near the bottom of the display, an electron beam which vertically corresponds to the aforementioned electron beam will not fall upon the aforementioned specific fluorescent layer color stripe. Thus, correct display of colors of an image in accordance with signals applied to the modulation electrodes will not be possible. This condition cannot be corrected by static adjustment of respective fixed voltage levels that are applied to the electrodes.
This is a serious practical problem, since such a rotational position deviation between the electrode assembly and the fluorescent layer stripe pattern of the CRT can very easily occur at the time of assembly of the electrodes within the outer envelope, resulting in the manufacturing yield of such a flat configuration CRT being reduced.